(As to CFRP)
CFRP is called in another more precise manner as CFRTS (Carbon Fiber Reinforced Thermo-Set Plastics) and is ordinarily a shaped article of resin having a matrix resin of thermo-set epoxy resin. While CFRP has been used recently in structures of aircrafts or automobiles, this owes to its mechanical properties of superlight weight and high strength. However, in the passenger aircraft B787 in which newest CFRP is used abundantly, rivet connection as a conventional fixing technique is employed for connecting a wing member of CFRP to another of CFRP and for connecting a CFRP member and an Al-alloy A7075 (extra-super-duralumin) member.
While CFRP contains bundles of carbon fiber (abbreviated as CF conveniently below), cloths of CF or the like abundantly, it is a shaped article of epoxy resin or thermoset plastic. Due to this, bolt and nut connection cannot be used for connecting CFRP with another member of a metal part or the like. Overfastening with a nut may break a CFRP member as a shaped article of resin. Assembling methods were studied for connecting various CRFP members each other regarding passenger aircrafts of the Boeing Company (USA), a manufacturer of aircrafts. After all, a method was employed such that through-holes are formed at the end of each CFRP member, rivets of Ti-alloy, of which the outer diameter is coincident with the through-holes, are pushed into the through-holes, and two members are connected for assembly.
However, it is not an easy matter to perform precision working of two kinds of CFRP members to be connected so that the diameters of the through-holes formed therein are in the level of “rivet diameter +0.1 mm”. Further, tremendous number of rivets is necessary for manufacturing an aircraft and the number of working steps required for forming through-holes is beyond supposition. If a metal alloy member can be joined with a CFRP member at the end of it with an adhesive to have high joining strength and reliable adhesiveness, wings or a body structure of an aircraft could be assembled easily by connecting one metal alloy member joined at the end side with another metal alloy member joined at the end side using bolt and nut. Further, if members, in each of which a CFRP part is joined with a metal alloy part, are formed and a metal alloy part of one member can be joined with the CFRP part of another member with high strength of adhesion, an aircraft could be obtained in which all parts are integrated by adhesion via metal alloy.
Here, in conventional techniques, it is clarified that a metal alloy subjected to specific surface treatment (see Patent Documents 1 to 9 by the present inventor for NAT (Nano Adhesion Technology) theory) can be joined with another member with extremely high joining strength of adhesion estimated by shear breaking strength of adhesion as a measurement of adhesive strength. However, while tensile breaking strength varies corresponding to metal species regarding tensile breaking strength (maximum tensile strength) of adhesion, clear reason of adhesion could not be explained yet. Further, theoretical explanation was not established yet as to what a level of maximum shear breaking strength or tensile breaking strength can be obtained using a specific one-part epoxy adhesive, and also as to the theory, that is, from what the strength comes. For this sake, it could not be decided what is to be improved further for making tensile strength of adhesion (maximum tensile strength) have a value near the maximum.
To say this regarding materials for an aircraft, it is necessary to develop a method for obtaining the maximum strength of composites of NAT type (composites obtained by adhesion according to NAT theory) of Al-alloy with Al-alloy, Ti-alloy with Ti-alloy and these with a CFRP member, and also to establish its theoretical explanation. However, there has not been sufficient development of a method or theoretical explanation for this yet. Techniques of accomplishing a light weight structure combining duralumin members, Ti-alloy members, CFRP members, or the like that are of light weight and rigid are important ones in times of energy-saving that will continue yet after now. If reliable adhesion techniques can be attained, it will form a core of such important techniques. Therefore, the above mentioned is important especially for moving machines such as aircrafts, automobiles, or the like.
(As to Current NAT Theory)
Here, summary of the conventional NAT theory established by the inventor will be explained. NAT theory concerns techniques for joining strongly a metal part with another metal member or a metal part with a resin part by use of an adhesive and the following conditions are required as regards metal materials, adhesives and adhesion steps. That is:
(1) The metal material has a roughened surface with convex-concave roughness of 0.8 to 10 μm (Rz) period (roughened surface of micron order period).
(2) Further, there is fine irregularities of 10 to 300 nm period on the above roughened surface.
(3) The surface layer having a surface formed with dual irregularities of the above (1) and (2) consists of at least one hard thin layer of metal oxide, metal phosphate and other ceramics.
(4) A one-part epoxy adhesive is used as an adhesive.
(5) In operation steps of adhesion, there is an operation step of “impregnation” as an operation step for causing the adhesive to penetrate into the bottom of the fine concaves of the irregularities on the surface of the metal member.
While NAT theory is one established by the inventor, it was at first a hypothesis (supposition). However, the inventor practiced the above (1) to (5) for various metal species and confirmed that adhesion strength attained with NAT theory usually exhibits values twice of data without NAT theory. From this, the inventor has come to recognize the hypothesis as a correct theory. Results demonstrating the recognition are disclosed in Patent Document 1 for aluminum alloy, Patent Document 2 for magnesium alloy, Patent Document 3 for stainless steel, Patent Document 4 for copper, Patent Documents 5 and 6 for titanium alloy, Patent Document 7 for general ferrous material, Patent Document 8 for aluminum-plated steel sheet and Patent Document 9 for zinc-plated steel sheet, respectively. Most of these have already been commercialized in the field of electronic machinery, moving machinery or the like, or are in the stage of provisional manufacturing before mass production.
(New NAT Theory)
Adhesive strength of a metal part with another metal part increased twice as before the above NAT theory, thus practical use of it is beginning as mentioned above. Further, it was necessary to change the method of measurement in order to measure the strong adhesion correctly. The inventor adopted a piece comprising two small pieces in which ends of two small pieces of 45 mm×15 mm×3 mm (thickness of 3 mm) are lapped each other and caused to adhere with an adhesive (adhesion area of 0.5 to 0.6 cm2) and used the piece comprising paired small pieces as a specimen for measuring “shear breaking strength of adhesion”, instead of the method of measurement by JISK 6849 (ISO 6922) “Method for testing tensile strength of adhesive.”
Similarly, as to a specimen for measurement of “tensile strength of adhesion (maximum tensile strength)”, small pieces of 18 mm×4 mm×3 mm were used at first, instead of following JIS 6850. The respective edge faces (4 mm×3 mm) of two of the small pieces were caused to confront each other and adhere with an adhesive to form a specimen for measurement, which was then subjected to tensile breaking for measurement of tensile breaking strength of adhesion. However, this specimen exhibited an inferiority such that there is much dispersion in data of measurement because bending moment is apt to be applied on the specimen depending on a manner of fastening, with chucks, the both gripped ends of specimen comprising two joined small pieces with total length of 36 mm when the specimen is to be pulled off in a tensile test. From this, configuration of specimens the inventor uses came to be of a small piece as 50 mm×10 mm×2 mm (the end face of 25 mm×3 mm is for adhesion), and then recently came to be of an elongated small piece as 100 mm×25 mm×3 mm (the end face of 25 mm×3 mm is for adhesion), which is decided to be favorable because dispersion of measured values becomes less.
In short, while NAT theory became spreading, “tensile strength of adhesion”, for which measurement is difficult among strengths of adhesion, has been left almost untreated without measuring in contrast to “shear strength of adhesion”, for which measurement is easy. Recent circumstances have changed this situation. NAT theory is superior not in adhesion of a metal member and another metal member, but also in adhesion of a metal member and a CFRP member, thus it has come to be remarked in industry of moving machines, especially by firms manufacturing automobiles, aircrafts, or the like. As derived by this, various official committees are inaugurated under the leadership of Ministry of Economics and Industry in Japan and working has begun for proposing new standard methods of ISO regarding methods for measuring strength of adhesion of joined articles with adhesives comprising metal parts, CFRP parts or the like.
As a result, concern of tensile strength of adhesion has become higher and the inventor has come to study improvement of the method for measuring tensile strength of adhesion that are to be submitted to ISO. While studying such, it became clear that what exhibits true strength of adhesion between a metal part and an adhesive is not shear breaking strength of adhesion but tensile breaking strength of adhesion, and that, strictly speaking about the surface configuration of the metal alloy material exhibiting the maximum tensile strength of adhesion, the surface configuration of the metal part according to NAT theory is not necessarily required. The inventor thought this as coming nearer to the essence of joining technology with adhesives and thought of establishing a new theory (New NAT Theory) by summing and arranging these matters. In short, it seemed that the New NAT theory that will be explained later will bring reliability of joining technology with adhesives to each of the above mentioned machine manufacturing industries.